Conventional aluminum alloys used for screw machine stock contain, among other alloying elements, lead (Pb). There is a growing health concern regarding the presence of lead in many materials, including the presence of lead in conventional aluminum alloy screw machine stock. As a result of these health concerns attempts were made to provide an aluminum alloy for screw machine stock which is not only essentially lead-free, but also exhibits physical properties that allows its ready use in lieu of the lead containing alloy. It has been found, that if the lead content of the conventional aluminum alloy screw machine stock is replaced with a substantially corresponding quantity of tin (Sn) and such Sn-containing aluminum alloy is then subjected to a thermomechanical treatment, an alloy is obtained which exhibits at least the equivalent, but in certain respects superior, physical properties to those exhibited by the Pb-containing aluminum screw machine stock alloy without encountering any significant health hazard which the prior art alloy created. The essentially lead-free aluminum alloy of the present invention contains from about 4.5 to about 6% Cu, a maximum of about 0.4% Si, a maximum of about 0.7% Fe, not more than about 0.3% Zn, from about 0.1 to about 1.0% Bi, from about 0.1 to about 0.5% Sn, balance Al and unavoidable impurities. The term "essentially lead-free", for the purposes of this invention, defines an alloy which contains lead only as an unavoidable impurity in amounts not exceeding 0.05%.
Use of tin in aluminum alloys employed for mechanical cutting operations, such as boring, drilling or lathe-cutting, has been known for many years. Thus, in U.S. Pat. No. 2,026,571 (Kempf et al) a free cutting aluminum alloy is described which contains copper, silicon and tin. The copper content of this known free cutting alloy is within the range of 3-12%, the silicon content is within the range of 0.5-2%, and the tin level is maintained within 0.005 to 0.1%. This prior art alloy may also contain 0.05 to 6% of one or more of the following elements: bismuth (Bi), thallium (Tl), cadmium (Cd) or lead (Pb). In order to improve the cutting properties of this alloy, the alloy may be subjected to a solution heat treatment and cold drawing.
Another prior art patent, U.S. Pat. No. 2,026,575 (Kempf et al), also discloses a free cutting aluminum alloy containing from about 4 to about 12% Cu, from about 0.01 to about 2% Sn, and about from about 0.05 to about 1.5% Bi. The patent mentions that to alter the physical properties of the alloy, it can be subjected to the usual heat treatments familiar to those skilled in the art of treating aluminum-copper alloys.
A more current reference, U.S. Pat. No. 5,122,208 (Alabi), discloses a wear-resistant and self-lubricating aluminum alloy which contains relatively substantial additions of tin and bismuth. This alloy has a Sn content from about 0.5 to about 3% with a corresponding quantity of Bi content. This alloy however has a very high silicon content and a very low Cu level which makes it unsuitable for use as a screw machine stock alloy. Sn and Bi-containing aluminum alloys were also employed in the manufacture of sacrificial anodes, however, the compositions of the conventional aluminum alloy sacrificial anodes make them unsuitable for use as screw machine stock.